Synthesis of Blue-Emitting CaMgSi 2 O 6 :Eu 21 Phosphor Using an Electrostatic Self-Assembly Deposition Method Tetsuya Kida, w Mohammed Mastabur Rahman, and Masamitsu Nagano Department of Chemistry and Applied Chemistry, Saga University, Saga, Japan Eu 21 -doped CaMgSi 2 O 6 phosphor was prepared by depositing mixed hydroxides of Ca, Mg, and Eu over spherical SiO 2 par- ticles (300 nm) pre-coated with polycations (polyethyleneimine), followed by calcination at 12001C in a reducing atmosphere. The prepared phosphor showed intense blue emission, ascribable to the 4f 7 -4f 6 5d transition of Eu 21 . In contrast, the luminescence intensity of the phosphor was considerably decreased when pre- pared without polycations. It was suggested that negatively charged hydroxides are deposited on positively charged SiO 2 surfaces pre-coated with polycations through electrostatic self- assembly interaction. On calcination, the hydroxide shells react with the SiO 2 cores to produce Eu 21 :CaMgSi 2 O 6 . I. Introduction S ILICATE-BASED phosphors have been widely used owing to their high luminescence efficiency and high chemical stabil- ity. For example, Mn 21 -doped Zn 2 SiO 4 has been utilized for fluorescent lamps, cathode ray tubes (CRTs), and plasma dis- play panels (PDPs). 1–3 Rare-earth-doped silicates such as Y 2 SiO 5 :Ce 31 are also important phosphors for the above applications. 1–3 Recently, Eu 21 -doped CaMgSi 2 O 6 (CMS) has attracted considerable attention as a new blue phosphor for PDPs. 4 This phosphor has been developed to overcome the drawbacks of the conventional blue phosphor, BaMgAl 10 O 17 : Eu 21 (BAM), which undergoes degradation by ion bombard- ment or ultraviolet radiation. 5–7 In addition, Eu 21 -doped CMS can also be used as a long phosphorescent phosphor when co- doped with Dy 31 . 8 Normally, phosphors are prepared by solid-state reactions (SSR) between raw materials at high temperatures. This route produces agglomerated particles of irregular shapes. However, such particles are not suitable for making phosphor screens ow- ing to their low packing density, which leads to a decrease in luminescence efficiency. In this study, CMS phosphors were prepared using an electrostatic self-assembly deposition (ESAD) method to control the morphology of the phosphor particles. The newly developed method we used here utilizes ESAD of precursor hydroxides onto colloidal particles as schematically shown in Fig. 1. Spherical SiO 2 particles with a narrow size dis- tribution are first coated with polycations to make their surfaces positively charged. Then, negatively charged Ca, Mg, and Eu hydroxide particles are deposited over the SiO 2 particles pre- coated with polycations via electrostatic interaction. The prepared core-shell structured particles are finally calcined to produce CMS phosphors by a reaction between the shells (mixed hydroxides) and the cores (SiO 2 ). Such processes using polycations or polyanions for preparing core-shell particles have recently been well exploited. 9–11 The distinct feature of the present method is that spherical SiO 2 particles with a narrow size distribution are used both as the morphology-determining template and as the precursor. Furthermore, it is expected that if the deposition of precursor hydroxides on SiO 2 particles is con- trolled and thus the spherical morphology of SiO 2 particles is retained, fine spherical powders with a narrow size distribution could be obtained. Such particles are preferable for the manu- facture of phosphor screens with good brightness and high res- olution owing to their improved packing density and low scattering of evolved light. In this study, we also attempted to prepare spherical CMS phosphor particles by the method we developed, and studied the effects of the calcination temperature on the morphology of the resulting particles. II. Experimental Procedure (1) Materials An aqueous solution of 30 wt% polyethyleneimine (PEI) {(CH 2 CH 2 NH) x : PEI, MW 5 50 000–100 000} was purchased from MP Biomedicals LLC (Irvine, CA). Tetraethyl orthosili- cate (TEOS), ethanol, Ca(NO 3 ) 2 , Mg(NO 3 ) 2, Eu(NO 3 ) 3 , and 28% NH 3 solutions were purchased from WAKO Pure Chem- ical Industries (Osaka, Japan). (2) Preparation of Spherical SiO 2 SiO 2 cores were prepared by hydrolysis and polycondensation of TEOS following a procedure of a modified Stober method. 12 Specifically, 42 mL absolute ethanol, 6.7 mL of NH 3 (28%), and 3.1 mL of TEOS were mixed and treated in an ultrasonic bath in a well-sealed glass flask for 30 min. The resulting dispersions contained spherical SiO 2 of 300 nm diameter, as confirmed by tranmission electron microscopy (TEM) analysis (Fig. 2). (3) Coating of Spherical SiO 2 with Polycations The spherical SiO 2 particles of 300 nm prepared were treated with a polyelectrolyte solution (PEI in water) under constant stirring to charge the SiO 2 surfaces positively. The particles were then washed several times with water by centrifugation to re- move excess polycations. This washing step is critical to prevent the aggregation of SiO 2 particles by polymer bridging, as noted by Chen and Somasundaram. 10 After washing and centrifuga- tion, the resultant PEI-coated SiO 2 particles were used for the preparation of CMS phosphors by the ESAD method. (4) Preparation of CMS Phosphors Eu 21 (5 mol%)-doped CaMgSi 2 O 6 (CMS) phosphors were pre- pared from PEI-coated spherical SiO 2 , Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , and Eu(NO 3 ) 3 at a stoichiometric molar ratio (Ca:Mg:- Si:Eu 5 1:1:2:0.05), as shown in Fig. 3. PEI-coated SiO 2 (0.006 mol) particles were re-dispersed in an aqueous solution (50 mL) containing nitrates of Ca (0.003 mol), Mg (0.003 mol), and Eu (0.00015 mol) (precursor nitrate solution). To cover the PEI- coated SiO 2 surfaces with mixed hydroxides of Ca, Mg, and Eu, an aqueous NH 3 (28%) solution (10 mL) was added to the above solution drop-by-drop under vigorous stirring until the pH of the solution reached 12. The products were then dried in J ournal J. Am. Ceram. Soc., 89 [5] 1492–1498 (2006) DOI: 10.1111/j.1551-2916.2006.00919.x r 2006 The American Ceramic Society 1492 J. Ballato—contributing editor This work was financially supported in part by the Sasagawa Scientific Research Grant, Japan (Project number 17-118). w Author to whom correspondence should be addressed. e-mail: kida@cc.saga-u.ac.jp Manuscript No. 20904. Received August 22, 2005; approved December 7, 2005.